From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-17.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 53CDAC4361B for ; Sat, 12 Dec 2020 10:05:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 27F23207C9 for ; Sat, 12 Dec 2020 10:05:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2438532AbgLLKEk (ORCPT ); Sat, 12 Dec 2020 05:04:40 -0500 Received: from mx2.suse.de ([195.135.220.15]:32870 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2438563AbgLLKE0 (ORCPT ); Sat, 12 Dec 2020 05:04:26 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1607765217; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=oopJDJgb5gj8qAschqVd0YtlbNXik5YiULD1RGyaUho=; b=NpUFyWPG/5kw2ZUY0Ui4vUEInBUVhPvDuN//AOLgKyW0lUox8lmEj4lDzIOWa5cWgweF1v v1UI4u2dpmM0uN5toBMOk8nezV59g9uwt5zoZkHiFZxyFjBdETRMJwe9/CqXWCbbmpgJ6W +fCDo9QkEJrzUL8wQI2AjgPMBpvYSVA= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 57428AEBE; Sat, 12 Dec 2020 09:26:57 +0000 (UTC) Subject: Re: [PATCH v2 12/18] btrfs: extent_io: implement try_release_extent_buffer() for subpage metadata support To: Qu Wenruo , Qu Wenruo , linux-btrfs@vger.kernel.org References: <20201210063905.75727-1-wqu@suse.com> <20201210063905.75727-13-wqu@suse.com> <047a66b1-5804-ac05-26ac-4eaf71f5c4df@suse.com> <00601760-4e7d-935a-e2bb-056593ff5a39@gmx.com> From: Nikolay Borisov Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= mQINBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/nSSP8YgfwoOqwrP+JR4pIqRK0AWWeWCSwmZ T7g+RbfPFlmQp+EwFWOtABXlKC54zgSf+uulGwx5JAUFVUIRBmnHOYi/lUiE0yhpnb1KCA7f u/W+DkwGerXqhhe9TvQoGwgCKNfzFPZoM+gZrm+kWv03QLUCr210n4cwaCPJ0Nr9Z3c582xc bCUVbsjt7BN0CFa2BByulrx5xD9sDAYIqfLCcZetAqsTRGxM7LD0kh5WlKzOeAXj5r8DOrU2 GdZS33uKZI/kZJZVytSmZpswDsKhnGzRN1BANGP8sC+WD4eRXajOmNh2HL4P+meO1TlM3GLl EQd2shHFY0qjEo7wxKZI1RyZZ5AgJnSmehrPCyuIyVY210CbMaIKHUIsTqRgY5GaNME24w7h TyyVCy2qAM8fLJ4Vw5bycM/u5xfWm7gyTb9V1TkZ3o1MTrEsrcqFiRrBY94Rs0oQkZvunqia c+NprYSaOG1Cta14o94eMH271Kka/reEwSZkC7T+o9hZ4zi2CcLcY0DXj0qdId7vUKSJjEep c++s8ncFekh1MPhkOgNj8pk17OAESanmDwksmzh1j12lgA5lTFPrJeRNu6/isC2zyZhTwMWs k3LkcTa8ZXxh0RfWAqgx/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABtCNOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuY29tPokCOAQTAQIAIgUCWIo48QIbAwYLCQgHAwIGFQgC CQoLBBYCAwECHgECF4AACgkQcb6CRuU/KFc0eg/9GLD3wTQz9iZHMFbjiqTCitD7B6dTLV1C ddZVlC8Hm/TophPts1bWZORAmYIihHHI1EIF19+bfIr46pvfTu0yFrJDLOADMDH+Ufzsfy2v HSqqWV/nOSWGXzh8bgg/ncLwrIdEwBQBN9SDS6aqsglagvwFD91UCg/TshLlRxD5BOnuzfzI Leyx2c6YmH7Oa1R4MX9Jo79SaKwdHt2yRN3SochVtxCyafDlZsE/efp21pMiaK1HoCOZTBp5 VzrIP85GATh18pN7YR9CuPxxN0V6IzT7IlhS4Jgj0NXh6vi1DlmKspr+FOevu4RVXqqcNTSS E2rycB2v6cttH21UUdu/0FtMBKh+rv8+yD49FxMYnTi1jwVzr208vDdRU2v7Ij/TxYt/v4O8 V+jNRKy5Fevca/1xroQBICXsNoFLr10X5IjmhAhqIH8Atpz/89ItS3+HWuE4BHB6RRLM0gy8 T7rN6ja+KegOGikp/VTwBlszhvfLhyoyjXI44Tf3oLSFM+8+qG3B7MNBHOt60CQlMkq0fGXd mm4xENl/SSeHsiomdveeq7cNGpHi6i6ntZK33XJLwvyf00PD7tip/GUj0Dic/ZUsoPSTF/mG EpuQiUZs8X2xjK/AS/l3wa4Kz2tlcOKSKpIpna7V1+CMNkNzaCOlbv7QwprAerKYywPCoOSC 7P25Ag0EWIoHPgEQAMiUqvRBZNvPvki34O/dcTodvLSyOmK/MMBDrzN8Cnk302XfnGlW/YAQ csMWISKKSpStc6tmD+2Y0z9WjyRqFr3EGfH1RXSv9Z1vmfPzU42jsdZn667UxrRcVQXUgoKg QYx055Q2FdUeaZSaivoIBD9WtJq/66UPXRRr4H/+Y5FaUZx+gWNGmBT6a0S/GQnHb9g3nonD jmDKGw+YO4P6aEMxyy3k9PstaoiyBXnzQASzdOi39BgWQuZfIQjN0aW+Dm8kOAfT5i/yk59h VV6v3NLHBjHVw9kHli3jwvsizIX9X2W8tb1SefaVxqvqO1132AO8V9CbE1DcVT8fzICvGi42 FoV/k0QOGwq+LmLf0t04Q0csEl+h69ZcqeBSQcIMm/Ir+NorfCr6HjrB6lW7giBkQl6hhomn l1mtDP6MTdbyYzEiBFcwQD4terc7S/8ELRRybWQHQp7sxQM/Lnuhs77MgY/e6c5AVWnMKd/z MKm4ru7A8+8gdHeydrRQSWDaVbfy3Hup0Ia76J9FaolnjB8YLUOJPdhI2vbvNCQ2ipxw3Y3c KhVIpGYqwdvFIiz0Fej7wnJICIrpJs/+XLQHyqcmERn3s/iWwBpeogrx2Lf8AGezqnv9woq7 OSoWlwXDJiUdaqPEB/HmGfqoRRN20jx+OOvuaBMPAPb+aKJyle8zABEBAAGJAh8EGAECAAkF AliKBz4CGwwACgkQcb6CRuU/KFdacg/+M3V3Ti9JYZEiIyVhqs+yHb6NMI1R0kkAmzsGQ1jU zSQUz9AVMR6T7v2fIETTT/f5Oout0+Hi9cY8uLpk8CWno9V9eR/B7Ifs2pAA8lh2nW43FFwp IDiSuDbH6oTLmiGCB206IvSuaQCp1fed8U6yuqGFcnf0ZpJm/sILG2ECdFK9RYnMIaeqlNQm iZicBY2lmlYFBEaMXHoy+K7nbOuizPWdUKoKHq+tmZ3iA+qL5s6Qlm4trH28/fPpFuOmgP8P K+7LpYLNSl1oQUr+WlqilPAuLcCo5Vdl7M7VFLMq4xxY/dY99aZx0ZJQYFx0w/6UkbDdFLzN upT7NIN68lZRucImffiWyN7CjH23X3Tni8bS9ubo7OON68NbPz1YIaYaHmnVQCjDyDXkQoKC R82Vf9mf5slj0Vlpf+/Wpsv/TH8X32ajva37oEQTkWNMsDxyw3aPSps6MaMafcN7k60y2Wk/ TCiLsRHFfMHFY6/lq/c0ZdOsGjgpIK0G0z6et9YU6MaPuKwNY4kBdjPNBwHreucrQVUdqRRm RcxmGC6ohvpqVGfhT48ZPZKZEWM+tZky0mO7bhZYxMXyVjBn4EoNTsXy1et9Y1dU3HVJ8fod 5UqrNrzIQFbdeM0/JqSLrtlTcXKJ7cYFa9ZM2AP7UIN9n1UWxq+OPY9YMOewVfYtL8M= Message-ID: Date: Sat, 12 Dec 2020 11:26:56 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <00601760-4e7d-935a-e2bb-056593ff5a39@gmx.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 12.12.20 г. 3:28 ч., Qu Wenruo wrote: > > > On 2020/12/12 上午12:57, Nikolay Borisov wrote: >> >> >> On 11.12.20 г. 14:11 ч., Qu Wenruo wrote: >>> >>> >>> On 2020/12/11 下午8:00, Nikolay Borisov wrote: >>>> >>>> >>>> On 10.12.20 г. 8:38 ч., Qu Wenruo wrote: >>>>> Unlike the original try_release_extent_buffer, >>>>> try_release_subpage_extent_buffer() will iterate through >>>>> btrfs_subpage::tree_block_bitmap, and try to release each extent >>>>> buffer. >>>>> >>>>> Signed-off-by: Qu Wenruo >>>>> --- >>>>>   fs/btrfs/extent_io.c | 73 >>>>> ++++++++++++++++++++++++++++++++++++++++++++ >>>>>   1 file changed, 73 insertions(+) >>>>> >>>>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c >>>>> index 141e414b1ab9..4d55803302e9 100644 >>>>> --- a/fs/btrfs/extent_io.c >>>>> +++ b/fs/btrfs/extent_io.c >>>>> @@ -6258,10 +6258,83 @@ void memmove_extent_buffer(const struct >>>>> extent_buffer *dst, >>>>>       } >>>>>   } >>>>> >>>>> +static int try_release_subpage_extent_buffer(struct page *page) >>>>> +{ >>>>> +    struct btrfs_fs_info *fs_info = >>>>> btrfs_sb(page->mapping->host->i_sb); >>>>> +    u64 page_start = page_offset(page); >>>>> +    int bitmap_size = BTRFS_SUBPAGE_BITMAP_SIZE; >>>> >>>> Remove this variable and directly use BTRFS_SUBPAGE_BITMAP_SIZE as a >>>> terminating condition >>>> >>>>> +    int bit_start = 0; >>>>> +    int ret; >>>>> + >>>>> +    while (bit_start < bitmap_size) { >>>> >>>> You really want to iterate for a fixed number of items so switch >>>> that to >>>> a for loop. >>> >>> The problem here is, it's not always fixed. >>> >>> If it finds one bit set, it will skip (nodesize >> sectorsize_bits) >>> bits. >>> >>> But if not found, it will skip to just next bit. >>> >>> Thus I'm not sure if for loop is really a good choice here for >>> differential step. >>> >>>> >>>>> +        struct btrfs_subpage *subpage; >>>>> +        struct extent_buffer *eb; >>>>> +        unsigned long flags; >>>>> +        u16 tmp = 1 << bit_start; >>>>> +        u64 start; >>>>> + >>>>> +        /* >>>>> +         * Make sure the page still has private, as previous run can >>>>> +         * detach the private >>>>> +         */ >>>> >>>> But if previous run has run it would have disposed of this eb and you >>>> won't find this page at all, no ? >>> >>> For the "previous run" I mean, previous iteration in the same loop. >>> >>> E.g. the page has 4 bits set, just one eb (16K nodesize). >> >> Isn't it guaranteed that if you iterate the eb's in a page if you meet >> an empty block then the whole extent buffer is gone, hence instead of >> doing bit_start++ you ought to also increment by the size of nodesize. >> >> For example, assume a page contains 4 EBs: >> >> 0|1|2|3|4|5|6|7|8|9|10|11|12|13|14|15 >> x|x|x|x|0|0|0|0|x|x| x|x |0 | 0|0 |0 >> >> So first bit is set, so you proceed to call release_extent_buffer on it, >> which clears the first 4 bits in tree_block_bitmap, in this case you've >> incremented by nodesize so next iteration begins at index 4. You detect >> it's unset (0) hence you increment it byte 1 and you repeat this for the >> next 3 bits, then you free the whole of the next eb. I argue that you >> also need to increment by nodesize in the case of a bit which is not >> set, because you cannot really see partially freed eb i.e you cannot see >> the following state: >> >> 0|1|2|3|4|5|6|7|8|9|10|11|12|13|14|15 >> x|x|x|x|x|0|0|0|x|x| x|x |0 | 0|0 |0 >> >> Am I missing something? > > It's not for partly freed eb, but nodesize unaligned eb. > > E.g. if we have a eb starts at sector 1 of a page, your nodesize based > iteration would go crazy. > Although we have ensured no subpage eb can cross page boundary, but it's > not the same requirement for nodesize alignment. > > Thus I uses the extra safe way for the empty bit. Which of course cannot happen because the allocator ensures that returned addresses are always aligned to fs_info::stripeize which in turn is always equal to sectorsize... So you add extra complexity for no apparent reason making code which is already subtle to be even more subtle.